Film deposition method and film deposition system for depositing a halogen compound film, and magnesium fluoride film

ABSTRACT

This invention provides film deposition method and film deposition system for depositing a halogen compound film, which are capable of depositing such a film while suppressing abuse that occurs due to deficiency of a halogen element even if the halogen element is dissociated from a film material. Specifically, the halogen compound film is deposited through a process including: evaporating a film material comprising a halogen compound by means of an evaporation source  3 ; ionizing the evaporated film material with a radio frequency power outputted from a radio frequency power supply unit  11  and supplied through a substrate holder  2 ; and causing the ionized film material to deposit on the substrate  5.  In this process a bias voltage outputted from a bias power supply unit  12  and applied to the substrate holder  2  causes halogen ions dissociated from ions of the halogen compound to be incorporated into the film being deposited on the substrate  5.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to film deposition method and filmdeposition system for depositing a halogen compound film.

[0002] Optical members such as optical lenses, displays and opticalcommunication members are formed at their surfaces with antireflectioncoating for lessening a loss in the amount of light or the like due toreflection. In the case where the substrate on which such antireflectioncoating is to be formed is glass, magnesium fluoride (MgF₂) is typicallyused as the surface layer of the antireflection coating. This isbecause: magnesium fluoride has a refractive index as low as 1.38 andhence is highly effective in antireflection; a film of magnesiumfluoride can be deposited easily by vacuum evaporation; such a magnesiumfluoride film has sufficient durability if deposited on a substrateheated to about 300° C.; and a like reason.

[0003] If the substrate is formed of a plastic, however, the vacuumevaporation cannot be employed because it is impossible to heat thesubstrate to such an elevated temperature.

[0004] In view of this, a method of forming a magnesium fluoride filmusing sputtering has been disclosed in Japanese Laid-Open PatentApplication Publication No. HEI 9-243802 for example. With thesputtering it is possible to form a dense and hard film on the surfaceof a substrate without the need of heating the substrate because theenergy of the film material popping out of the target in the sputteringis higher than that in the case of the vacuum evaporation.

[0005] In the formation of a magnesium fluoride film by sputtering,however, fluorine is dissociated from the film material (magnesiumfluoride) due to impact of ions when the film material pops out of thetarget, thus resulting in a fluorine-deficient film exhibiting anincreased light absorption on the surface of the substrate. To avoidthis inconvenience the aforementioned film-forming method controlsheating of the target to a predetermined high temperature to allow thefilm material to pop out of the target with its molecular state kept asit is. By so doing the dissociation of fluorine from the film materialis prevented, with the result that an increase in light absorption issuppressed.

[0006] Thus, the film-forming method employing the sputtering mustcontrol the temperature of the target to allow the film material to popout of the target with its molecular state kept as it is.

[0007] Such a problem arises commonly in forming not only magnesiumfluoride films but also halogen compound films for use as variousoptical thin films such as antireflection coating and half mirrorcoating by the use of the sputtering.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention has been made to resolve the foregoingproblem and intends to provide a film deposition method and a filmdeposition system for depositing a halogen compound film, which arecapable of depositing such a film while suppressing adverse effect thatoccur due to deficiency of a halogen element if the halogen element isdissociated from a film material.

[0009] With a view to resolving the foregoing problem, the presentinvention provides a film deposition method for depositing a halogencompound film, comprising the steps of: placing a substrate on anobverse side of a bias supply electrode disposed in a vacuum chamber;evaporating a film material comprising a halogen compound; supplying aradio frequency voltage to the bias supply electrode used as one ofelectrodes to generate plasma in the vacuum chamber; and applying to thebias supply electrode a bias voltage varying in the form of a wavehaving a negative mean value and a maximum value exceeding a self-biascaused by the radio frequency voltage, whereby the evaporated filmmaterial is ionized and deposited on the substrate to form the halogencompound film on the substrate.

[0010] This method according to the present invention makes it possibleto ionize a halogen compound and deposit the ionized film material on asubstrate to deposit a film. Even if ions of the halogen element aredissociated from ions of the halogen compound, it is possible to guidesuch dissociated ions of the halogen element to the substrate and allowsuch ions to be incorporated into the film to be formed on the substrateby application of the bias voltage varying in the form of a wave havinga negative mean value and a maximum value exceeding a self-bias causedby the radio frequency voltage to the bias supply electrode.

[0011] Thus, a halogen element, which is easy to dissociate in the formof negative ions, can be incorporated into an intended film being formedthrough ionization of a halogen compound and, hence, it is possible toprevent the resulting film from being deficient in the halogen element.Further, since the film formation is based on ion plating, it ispossible to deposit a film rendered dense.

[0012] The maximum value of the bias voltage may be a positive voltage.With this feature it is possible to lessen the deficiency in the halogenelement.

[0013] The bias voltage may be applied by a power supply.

[0014] The bias voltage applied may be based on a voltage generated in amatching circuit provided for matching an impedance on a power supplyside associated with the radio frequency voltage to that on a vacuumchamber side. With this feature it is possible to dispense with adedicated bias power supply.

[0015] In the formation of a film based on ion plating which ionizes afilm material to generate plasma by means of a radio frequency power, anevaporation source is provided for evaporating the film material, whilea coiled ionization electrode is disposed between the evaporation sourceand the substrate for supplying the radio frequency power, whereby thefilm material evaporated by the evaporation source can be ionized bybeing passed through the coiled ionization electrode. Since thedeposition of a film according to this feature of the present inventionis also based on ion plating, it is also possible to deposit the filmrendered dense.

[0016] In ionizing the film material in the film formation according tothe present invention the film material comprising the halogen compoundmay be evaporated and ionized in a clustered fashion by cluster iongenerating means. This feature allows clustered ions to impinge upon thesubstrate thereby depositing the film. The application of the biasvoltage assuming the aforementioned positive voltage to the bias supplyelectrode allows ions of the halogen element dissociated from theclustered ions of the halogen compound to be incorporated into the filmbeing deposited on the substrate.

[0017] It is possible to employ an arrangement wherein a substrateholder holding the substrate on its back surface in the vacuum chamberis formed of an electrically-conductive material and is used also as thebias supply electrode. This arrangement makes it possible to apply thebias voltage to the substrate holder holding the substrate to bedeposited with the film on its back surface and, hence, the bias voltagefor causing ions to be incorporated into the film being deposited on thesubstrate can be applied easily and efficiently.

[0018] In the foregoing film deposition method it is possible that anelectron beam evaporation source is further provided comprising acrucible holding a material same as the halogen compound to beevaporated, an electron gun for evaporating the halogen compound held inthe crucible by heating with an electron beam, and a shutter spaced apredetermined distance from the crucible in a direction toward thesubstrate, and that the halogen compound in the crucible is evaporatedby heating with the electron beam, while direct impingement of thehalogen compound thus evaporated upon the substrate is obstructed withthe shutter, whereby the evaporated halogen compound is guidedsidewardly of the shutter and the crucible.

[0019] Since the shutter obstructs the flow of the halogen compoundevaporated by the electron beam evaporation source, it is impossible forthe evaporated halogen compound to impinge directly upon the substrate.Ions of the halogen element dissociated from the evaporated halogencompound obstructed by the shutter and guided sidewardly of the shutterand the crucible are guided toward the substrate by the bias voltageassuming a positive voltage.

[0020] Thus, the amount of ions of the halogen element to be supplied tothe substrate can be increased relative to the amount of the evaporatedhalogen compound and, hence, it is possible to avoid a relative decreasein the amount of the halogen contained in the film to be deposited onthe substrate.

[0021] Further, since the electron beam evaporation source evaporatesthe halogen compound by means of the electron beam, the halogen compoundcan be decomposed more finely, which facilitates the dissociation ofions of the halogen element. This makes it possible to supply such ionsof the halogen element to the substrate more easily.

[0022] The aforementioned halogen compound may be magnesium fluoride(MgF₂). Magnesium fluoride can be formed into an optical thin film. Informing a magnesium fluoride film according to the present invention itis possible to prevent the magnesium fluoride film from becomingdeficient in a halogen element. Thus, the present invention is capableof forming a magnesium fluoride film without impairing optical functionsuch as transparency.

[0023] The aforementioned bias voltage may have a frequency rangingbetween 100 kHz and 2.45 GHz. With the bias voltage having such afrequency the halogen element can advantageously be incorporated intothe film and prevented from being eliminated from the film.

[0024] According to the present invention, there is also provided anapparatus for depositing a halogen compound film, comprising: a vacuumchamber; a bias supply electrode disposed in the vacuum chamber andhaving an obverse side for receiving a substrate thereon; an evaporationsource for evaporating a film material of the halogen compound film tobe deposited on the substrate; a radio frequency power supply forsupplying a radio frequency voltage to the bias supply electrode used asone of electrodes to generate plasma in the vacuum chamber; and a biaspower supply for applying to the bias supply electrode a bias voltagehaving a frequency ranging between 100 kHz and 2.45 GHz and varying inthe form of a wave having a negative mean value and a positive maximumvalue. This apparatus is capable of incorporating the halogen elementinto the film and preventing elimination of the halogen element from thefilm.

[0025] According to the present invention, there is also provided amagnesium fluoride film obtainable by a method comprising the steps of:placing a substrate on an obverse side of a bias supply electrodedisposed in a vacuum chamber; evaporating magnesium fluoride; supplyinga radio frequency voltage to the bias supply electrode used as one ofelectrodes to generate plasma in the vacuum chamber; and applying to thebias supply electrode a bias voltage having a frequency ranging between100 kHz and 2.45 GHz and varying in the form of a wave having a negativemean value and a positive maximum value, thereby depositing themagnesium fluoride film on the substrate. The magnesium fluoride filmthus obtained is a dense and hard film having a lowered absorption inthe visible light region.

[0026] The magnesium fluoride film according to the present inventionhas a crystal grain diameter not less than 3 nm and not more than 10 nm.

[0027] The above and further objects and features of the invention willmore fully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0028]FIG. 1A is a schematic overview of the configuration of a vacuumdeposition system capable of practicing the present invention;

[0029]FIG. 1B is a diagram showing an example of a bias voltageaccording to the present invention;

[0030]FIG. 2 is a diagram showing another example of the bias voltageaccording to the present invention;

[0031]FIGS. 3A to 3C are views showing other examples of theconfiguration of a bias power supply unit according to the presentinvention;

[0032]FIG. 4 is a schematic view showing the configuration of a vacuumdeposition system capable of practicing the present invention;

[0033]FIG. 5 is a schematic view showing the configuration of a vacuumdeposition system capable of practicing the present invention;

[0034]FIG. 6 is a schematic view showing the configuration of a vacuumdeposition system capable of practicing the present invention;

[0035]FIG. 7 is a graph showing the influence of a radio frequency powerupon the absorption coefficient of a magnesium fluoride thin film;

[0036]FIG. 8 is a graph showing the influence of a substrate temperatureupon the absorption coefficient of a magnesium fluoride thin film;

[0037]FIG. 9A is a schematic view illustrating obstruction toimpingement of fluorine ions upon a substrate surface;

[0038]FIG. 9B is a schematic view illustrating elimination of fluorineions from a substrate surface;

[0039]FIG. 10A is a view illustrating an action of the presentinvention, specifically the action of incorporating dissociated fluorineions;

[0040]FIG. 10B is a view illustrating an action of the presentinvention, specifically the action of preventing elimination of fluorineions;

[0041]FIG. 11 is a graph showing the dependence of the absorptivity of amagnesium fluoride thin film upon the pulse frequency of a bias voltage;

[0042]FIG. 12 is a graph showing the absorptivity of a magnesiumfluoride thin film deposited on a quartz substrate within the visiblelight region;

[0043]FIG. 13A is a perspective view schematically showing the overviewof a wear-resistance tester for illustrating a wear-resistance test;

[0044]FIG. 13B is a table showing wear-resistance evaluation criteriafor illustrating the wear-resistance test;

[0045]FIG. 14 is a table showing the crystal grain diameters ofmagnesium fluoride films;

[0046]FIG. 15A is a view showing the structure of a multi layered filmfor illustrating an application of an embodiment according to apracticing the present invention to a multi layered film;

[0047]FIG. 15B is a graph showing the reflectivity of a multi-layeredfilm formed on a substrate within the visible light region forillustrating an application of an embodiment according to a practicingthe present invention to a multi layered film;

[0048]FIG. 16 is a schematic view showing an example of theconfiguration of a vacuum deposition system that is suitable for a biasvoltage having a higher pulse frequency;

[0049]FIG. 17 is a graph showing variations in the electric potential ofthe substrate holder shown in FIG. 1; and

[0050]FIG. 18 is a graph showing variations in the electric potential ofthe substrate holder shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0051] Hereinafter, modes of practicing the present invention will bedescribed with reference to FIGS. 1 to 18.

[0052]FIG. 1A is a schematic overview of the configuration of a filmdeposition system 10 as one example of such an system capable ofpracticing the present invention. The film deposition system 10 isconfigured to deposit films on the basis of ion plating as a filmdeposition method.

[0053] The film deposition system 10 includes a vacuum chamber 1 and apower supply unit 8 comprising a radio frequency (RF) power supply unit11 and a bias power supply unit 12.

[0054] In an upper portion of the inside of the vacuum chamber 1 isdisposed a substrate holder 2 for holding a substrate 5 from its reverseside opposite from its obverse side on which a film is to be deposited.In the example shown in FIG. 1A the substrate holder 2 is formed of anelectrically-conductive material so that an electric power can besupplied into the vacuum chamber 1. As will be described later, thesubstrate holder 2 is adapted to be capable of functioning as a biassupply electrode as well as of supplying a radio frequency power.

[0055] Further, the substrate holder 2 can be driven for rotatably bymeans of a motor not shown and, hence, a film can be deposited on thesubstrate 5 rotated by the substrate holder 2 rotating.

[0056] In a lower portion of the inside of the chamber 1 is disposed anevaporation source 3 for holding a film material comprising a halogencompound and evaporating it within the space defined in the chamber 1.As the evaporation source 3 it is possible to use any one of variousevaporation sources that are capable of evaporating a film material inthe space defined in the chamber 1, including one adapted to evaporatethe film material through resistive heating by a heating power supply,one adapted to evaporate the film material through heating with anelectron gun, one adapted to evaporate the film material by sputtering,and one adapted to evaporate the film material by arc discharge.

[0057] The vacuum chamber 1 is provided with evacuation means such as avacuum pump and gas supply means, which are not particularly shown, forhaving a desired vacuum atmosphere therein. That is, the atmospherewithin the vacuum chamber 1 can be adjusted to a desired vacuumatmosphere meeting film-depositing conditions established.

[0058] Further, the vacuum chamber 1 is formed of anelectrically-conductive material and has a chamber wall grounded.

[0059] The radio frequency power supply unit 11 supplies an electricpower for generating plasma in the chamber 1 to ionize (excite) the filmmaterial evaporated from the evaporation source 3. The radio frequencypower supply unit 11 has one output terminal connected to the substrateholder 2 through a high pass filter 15 and other output terminalgrounded. The radio frequency power outputted from the radio frequencypower supply unit 11 is applied to the substrate holder 2.

[0060] The high pass filter 15, which is located between the radiofrequency power supply unit 11 and the substrate holder 2, permits thepower outputted from the radio frequency power supply unit 11 to passtherethrough toward the substrate holder 2 side while blocking theinputting of the power outputted from the bias power supply unit 12 tothe radio frequency power supply unit 11.

[0061] As to the specific power value and frequency of the poweroutputted from the radio frequency power supply unit 11, desired powervalue and frequency are determined to meet the kind of the material of afilm to be deposited and film depositing conditions.

[0062] Between the radio frequency power supply unit 11 and the highpass filter 15 is disposed a matching box not particularly shown. Thematching box includes a well-known matching circuit comprising acapacitor and a coil. Adjustment of the matching box allows matching tobe made between the impedance of the radio frequency power supply unit11 side and that of the vacuum chamber 1 side.

[0063] The bias power supply unit 12 comprises a waveform generator 13and a bias power supply 14. The waveform generator 13 is adapted togenerate a waveform of a bias voltage to be outputted from the biaspower supply unit 12.

[0064] The waveform generator 13 is capable of generating variouswaveforms such as DC component steadily assuming a constant value, ACcomponents of respective frequencies, square waves and triangular wavesas basic components. The waveform generator 13 is also capable ofcomposing a basic waveform based on plural basic components. The biaspower supply 14 outputs a bias voltage amplified to a predeterminedmagnitude of power based on such a basic waveform generated by thewaveform generator 13.

[0065] The bias power supply 14 has one output terminal connected to thesubstrate holder 2 through a first low pass filter 16 and other outputterminal grounded. The bias voltage outputted from the bias power supply14 is applied to the substrate holder 2.

[0066] The first low pass filter 16, which is located between the biaspower supply 14 and the substrate holder 2, permits the power outputtedfrom the bias power supply 14 to pass therethrough toward the substrateholder 2 side while blocking the inputting of the power outputted fromthe high-frequency power unit 11 to the bias power supply unit 12.

[0067] The following description is directed to the bias voltageoutputted from the bias power supply unit 12. FIG. 1B shows an exampleof a bias voltage waveform. In FIG. 1B the horizontal axis correspondsto time (sec.) while the vertical axis corresponds to the magnitude of avoltage value (V). The upper side above the horizontal axis is apositive voltage zone, while the lower side below the horizontal axis isa negative voltage zone.

[0068] As shown in FIG. 1B, the bias voltage comprises positive biasportions assuming a constant positive voltage value (V_(P1)) andnegative bias portions assuming a constant negative voltage value(−V_(B1)).

[0069] For duration T_(W1) of one period (T_(W1)+T₁) a positive voltagein the form of a square wave pulse is outputted as the bias voltage asshown in FIG. 1B. For the other duration T₁ of one period (T_(W1)+T₁) anegative voltage is outputted as the bias voltage as shown in FIG. 1B.

[0070] With use of the film deposition system 10 described above a filmcan be deposited in the following manner. The evaporation source 3 isloaded with a film material comprising a halogen compound, and thesubstrate 5 is set on the substrate holder 2. In setting the substrate 5on the substrate holder 2 the substrate 5 is placed so that its obverseside to be deposited with the film faces opposite to the evaporationsource 3.

[0071] Subsequently, the power supply unit 8 is actuated to supply theradio frequency power into the chamber 1 through the substrate holder 2and apply the bias voltage to the substrate holder 2, while the halogencompound is evaporated by the evaporation source 3.

[0072] By so doing, plasma is generated in the vacuum chamber 1, and thehalogen compound evaporated from the evaporation source 3 is ionized(excited) by plasma thus generated. Resulting ions of the halogencompound are caused to impinge upon and adhere to the substrate 5, thusdepositing the film.

[0073] In the process of depositing a film on the substrate 5 with thefilm deposition system 10, if the radio frequency power is supplied tothe chamber 1 in the presence of plasma therein, a negative potential isprovided the obverse side of the substrate 5 due to what is called a“self-bias”.

[0074] Such a negative potential due to the self-bias and the negativebias caused by the bias voltage make it possible to accelerate themovement of positively-charged ions of the halogen compound toward thesubstrate 5. In this way the negative bias of the bias voltage enablesfurther acceleration of the movement of such halogen compound ions, thusrendering denser the structure of the film to be deposited on thesubstrate.

[0075] Though the halogen element with weak chemical bond is easilydissociated from the ionized halogen compound in the film-depositingprocess performed by the film deposition system 10, the presentinvention allows the halogen element once dissociated to be incorporatedinto the film being deposited on the substrate 5. Specifically, thepositive bias of the bias voltage allows negatively-charged ions of thehalogen element to be incorporated into the film being deposited on thesubstrate 5.

[0076] Thus, the film to be deposited on the substrate 5 can beprevented from becoming deficient in the halogen element and, hence, itis possible to avoid any degradation in the function of the film whichwould otherwise occur due to the deficiency of the halogen element.

[0077] Here, a brief description is made of the self-bias. The high passfilter 15 has a blocking capacitor (not shown) serially connected to theradio frequency power supply unit 11. This blocking capacitor permitsthe radio frequency component of a current to pass therethrough butblocks the DC component of the current. Accordingly, when the radiofrequency power is supplied into the vacuum chamber 1, electric chargeintroduced into the substrate holder 2 from plasma generated by theradio frequency power is built up in the blocking capacitor. Sinceelectrons present in plasma travel toward the substrate holder 2 at ahigher speed than do ions present in plasma, an offset voltage, which isdetermined by the capacity of the blocking capacitor and the amount ofcharge in the capacitor, is generated between the opposite ends of theblocking capacitor and is applied to the substrate holder 2. A voltagegenerated at an electrode (the substrate holder 2 in this case) incontact with plasma according to this mechanism is referred to as a“self-bias”. Such a self-bias is substantially a constant voltage(substantially a DC voltage) and is generated so that the substrateholder 2 assumes a negative potential.

[0078] The relationship between the self-bias and the bias voltageoutputted from the bias power supply unit 12 is described below. Theblocking capacitor and the bias power supply 12 are connected inparallel with each other with respect to the substrate holder 2. In sucha case one of the self-bias and the bias voltage outputted from the biaspower supply unit 12, whichever the superior, is predominantly appliedto the substrate holder 2. In the subject mode of practicing the presentinvention the bias voltage outputted from the bias power supply unit 12is superior to the self-bias and hence is predominantly applied to thesubstrate holder 2. Thus, as shown in FIG. 17, potential V_(H) at thesubstrate holder 2 substantially conforms with the bias voltage (referto FIG. 1B) generated by the bias power supply unit 12 and varies asdoes the bias voltage.

[0079] Maximum value V_(P1) of the bias voltage generated by the biaspower supply unit 12 is set higher than the voltage value (negative) ofa self-bias that would be generated if the bias power supply unit 12 isabsent. By so doing it is possible to suppress an electrical repulsioncaused by the self-bias against negatively-charged ions of the halogenelement thereby to facilitate incorporation of such halogen element ionsinto the film being deposited on the substrate 5. Assuming that theabsolute value of the self-bias that would be generated if the biaspower supply unit 12 is absent is V_(dc), the maximum value V_(P1) ofthe bias voltage preferably within the range of from +0.5V_(dc) to−0.5V_(dc). The absolute value V_(dc) of the self-bias is about 200 V toabout 300 V under typical film-forming conditions. What should be notedhere is that the maximum value V_(P1) of the bias voltage need notnecessarily assume a positive value as shown in FIG. 1B and hence mayassume a negative value. Of course, it is desirable that the maximumvalue V_(P1) of the bias voltage be as high as possible within theaforementioned range. This is because such a high maximum value allowsions of the halogen element to be incorporated into the film beingdeposited on the substrate 5 easily. However, a maximum value V_(P1)more than the upper limit of the aforementioned range is not preferablebecause discharge within the vacuum chamber 1 becomes unstable. Thewidth Δt of a positive-direction pulse of the bias voltage is preferably10 μs or less. If it is more than 10 μs, discharge within the chamber 1becomes unstable. With the film deposition method 10 shown in FIG. 1Athe frequency of positive-direction pulses of the bias voltage ispreferably within the range of from 100 kHz to 4×13.56 MHz. As will bedescribed later, when the frequency of positive-direction pulses is 100kHz or more, the effect of superposition of positive-direction pulsesbecomes noticeable, while if it is more than 4×13.56 MHz, dischargewithin vacuum the chamber 1 of the film deposition system 10 employingthe radio frequency power supply unit 12 as shown in FIG. 1A becomesunstable.

[0080] Another example of the bias voltage is described below withreference to FIG. 2. FIG. 2 shows another waveform example of the biasvoltage. The bias voltage shown in FIG. 2 has a gently curved waveformas a whole and comprises sinusoidal-wave negative bias portions assuminga negative voltage value (−V_(B2)) and sinusoidal-wave positive biasportions assuming a positive voltage value (V_(P2)).

[0081] As shown in FIG. 2, a positive voltage is outputted as the biasvoltage for the duration T_(W2) of one period (T_(W2)+T₂), while anegative voltage is outputted for the other duration T₂. The biasvoltage of the waveform shown in FIG. 2 can be obtained by adding a DCvoltage assuming a constant negative voltage value to an AC voltage.

[0082] The positive bias portions of the bias voltage having thewaveform shown in FIG. 2 have a mean voltage value of a magnitude setlarger than at least the absolute value of a negative potential at thesubstrate 5 caused by the aforementioned self-bias. By so setting it ispossible to suppress electrical repulsion caused by the self-biasagainst negatively-charged ions of the halogen element thereby tofacilitate incorporation of such halogen element ions into the filmbeing deposited on the substrate 5.

[0083] It is desirable that the mean value of the magnitudes of positivebias portions be larger than the absolute voltage value of a negativepotential caused by the aforementioned self-bias. If so, it is possibleto provide a positive potential at the substrate 5 thereby to causehalogen element ions to be incorporated into the film being deposited onthe substrate 5 more easily.

[0084] Next, another example of the power supply unit that can be usedin the aforementioned film deposition system 10 will be described withreference to FIGS. 3A to 3A. A bias power supply unit 22 shown in FIG.3A comprises a DC power supply 24 capable of outputting a constantnegative DC voltage for providing a negative bias, and a pulse powersupply 23 capable of outputting a constant positive pulse voltage forproviding a positive bias.

[0085] The power outputted from the DC power supply 24 and the poweroutputted from the pulse power supply 23 form a bias voltage, which inturn is outputted to the substrate holder 2 side through a first lowpass filter 21.

[0086] The first low pass filter 21 permits the bias voltage outputtedfrom the DC power supply 24 and the pulse power supply 23 to passtherethrough toward the substrate holder 2 side while blocking theinputting of the power outputted from the radio frequency power unit 11to the bias power supply unit 22 side.

[0087] In the bias power supply unit 22 configured as shown in FIG. 3Athe DC power supply 24 for providing a negative bias and the pulse powersupply 23 for providing a positive bias are independent of each otherand, hence, it is possible to adjust the negative bias and the positivebias separately with ease.

[0088]FIG. 3B shows yet another example of the bias power supply unit.Bias power supply unit 26 shown in FIG. 3B comprises a DC power supply27 capable of outputting a constant negative DC voltage for providing anegative bias, and an impulse train power supply 28 for providing apositive bias. The DC power supply 27 outputs a power to the substrateholder 2 side through a second low pass filter 29, while the impulsetrain power supply 28 outputs a power to the substrate holder 2 sidethrough a band pass filter 30. These powers thus outputted form a biasvoltage to be applied to the substrate holder 2.

[0089] The second low pass filter 29, which is located between the DCpower supply 27 and the substrate holder 2, permits the power outputtedfrom the DC power supply 27 to pass therethrough toward the substrateholder 2 side while blocking the inputting of respective powersoutputted from the radio frequency power unit 11 and from the impulsetrain power supply 28 to the DC power supply 27 side.

[0090] The band pass filter 30, which is located between the impulsetrain power supply 28 and the substrate holder 2, permits the poweroutputted from the impulse train power supply 28 to pass therethroughtoward the substrate holder 2 side while blocking the inputting ofrespective powers outputted from the radio frequency power unit 11 andfrom the DC power supply 27 to the impulse train power supply 28 side.

[0091] In the bias power supply unit 26 shown in FIG. 3B the DC powersupply 27 for providing a negative bias and the impulse train powersupply 28 for providing a positive bias are independent of each other,and similarly the filters 29 and 30 associated with respective powersoutputted from these power supplys to the substrate holder 2 side areindependent of each other. Hence, it is possible to adjust the biasvoltage more easily.

[0092]FIG. 3C shows yet another example of the power supply unit.According to this example shown in FIG. 3C, the power supply unitcomprises a linear amplifier 31 and a function generator 32. Thefunction generator 32 generates a waveform composed of the radiofrequency waveform of the radio frequency power, the waveform ofnegative bias portions of the bias voltage and the waveform of positivebias portions of the bias voltage and causes the linear amplifier 31 toamplify the waveform thus composed and output the waveform amplified asa desired output to the substrate holder 2 side.

[0093] The power supply unit configured as shown in FIG. 3C does notrequire the provision of a filter and hence dispenses with adjustment ofsuch a filter. Further, it is possible to unify controls over radiofrequency outputs and negative and positive bias outputs of the biasvoltage thereby to adjust the power balance easily.

[0094] It should be noted that in the case of the bias power supply unit22 or 26 shown in FIG. 3A or 3B which is adapted to output a negativebias portion and a positive bias portion independently of each other,the outputs of the DC power supply 24 or 27 and pulse power supply 23 orimpulse train power supply 28 are adjusted relative to each other sothat the bias voltage composed of these negative and positive biasoutputs can provide a negative bias of a predetermined voltage value anda positive bias of a predetermined voltage value.

[0095] Though the foregoing description has been directed to an exampleof the bias voltage that provides both a positive bias and a negativebias, the negative bias need not necessarily be outputted and it ispossible to exert an electrical attractive force upon ions of thehalogen compound thereby to accelerate the movement of such ions towardthe substrate 5 without the need of outputting the negative bias.Specifically, since the aforementioned self-bias caused by the supply ofthe radio frequency power to plasma assumes a negative potential, theself-bias is capable of exerting an attractive force upon ions of thehalogen compound.

[0096] In practicing the present invention with the film depositionsystem 10 described above the film deposition system 10 may beconfigured so that it is controlled through a controller not shown.

[0097] This controller is capable of exercising controls over the filmdeposition system 10 so that: the aforementioned radio frequency powersupply unit 11 operates to output a radio frequency power having adesired power value and a desired frequency; the aforementioned biaspower supply unit 12, 22 or 26 operates to output desired negative biasand positive bias; and the aforementioned function generator 32 andlinear amplifier 31 perform desired actions.

[0098] The controller may be configured to actuate the evacuation meanssuch as a vacuum pump and the gas supply means so as to provide adesired vacuum atmosphere in the chamber 1.

EXAMPLE

[0099] The present invention can use various halogen compounds as filmmaterials for films that can be deposited according to the presentinvention. In the subject example the present invention is applied tomagnesium fluoride as a most preferred application of the presentinvention.

[0100] First, the behavior of fluorine ions in ion plating is describedbelow.

[0101]FIG. 7 is a graph showing the influence of a radio frequency powerupon the absorption coefficient of a magnesium fluoride thin film. InFIG. 7 the horizontal axis and the vertical axis represent a wavelengthand an absorption coefficient, respectively. The radio frequency powerwas varied stepwise to assume three levels of 50 W, 300 W and 500 W as aparameter. As apparent from FIG. 7, the absorption coefficient of amagnesium fluoride thin film deposited on a substrate in the visiblelight region increases as the radio frequency power supplied to a vacuumchamber becomes higher.

[0102]FIG. 8 is a graph showing the influence of a substrate temperatureupon the absorption coefficient of a magnesium fluoride thin film. InFIG. 8 the horizontal axis and the vertical axis represent a wavelengthand an absorption coefficient, respectively. The substrate temperaturewas varied stepwise to assume seven levels of not heated, 50° C., 100°C., 150° C., 200° C., 250° C., 300° C. as a parameter. The radiofrequency power was set to 300 W. As apparent from FIG. 8, theabsorption coefficient of a magnesium fluoride thin film deposited on asubstrate in the visible light region increases as the substratetemperature rises.

[0103]FIGS. 9A and 9B are schematic views illustrating obstruction toimpingement of fluorine ions upon a substrate surface and elimination offluorine ions from a substrate surface; that is, FIG. 9A illustrates theobstruction to impingement, while FIG. 9B illustrates the elimination.

[0104] The reason that the absorption coefficient of a magnesiumfluoride thin film deposited on a substrate increases as the radiofrequency power becomes higher, is considered as follows. As shown inFIG. 9A, the surface of the substrate 5 was negatively charged due to aself-bias caused by a radio frequency voltage. On the other hand,magnesium fluoride 101 evaporated from the evaporation source and passedthrough plasma 100 is positively charged. At this time fluorine ions 103are dissociated from a portion of the magnesium fluoride 101. Sincethese fluorine ions 103 are negatively charged, their impingement uponthe substrate 5 is obstructed. As a result, the magnesium fluoride thinfilm deposited on the substrate 5 is in a fluorine-deficient conditionand hence has an increased absorption coefficient in the visible lightregion. The amount of fluorine ions 103 dissociated from the magnesiumfluoride 101 is proportional to the radio frequency power. For thisreason the absorption coefficient of the magnesium fluoride thin filmincreases as the radio frequency power becomes higher.

[0105] The reason that the absorption coefficient of a magnesiumfluoride thin film increases as the substrate temperature rises, isconsidered as follows. As shown in FIG. 9B, since the substrate surfaceis negatively charged due to a self-bias caused by a radio frequencyvoltage, fluorine of magnesium fluoride 101 once deposited on thesurface of the substrate 5 undergoes electrical repulsion. Since thebonding power of fluorine in magnesium fluoride 101 is weak, fluorineions 103 are eliminated from a portion of magnesium fluoride 101. Thenumber of eliminated fluorine ions 103 is proportional to thetemperature of the substrate 5. For this reason the absorptioncoefficient of the magnesium fluoride thin film increases as thesubstrate temperature rises.

[0106] The actions and effects of the present invention are described indetail below.

[0107]FIGS. 10A and 10B are views illustrating actions of the presentinvention; specifically, FIG. 10A illustrates the action ofincorporating dissociated fluorine ions, while FIG. 10B illustrates theaction of preventing elimination of fluorine.

[0108] Since the bias voltage has a duration for which a positive biasis provided, fluorine ions 103 dissociated from magnesium fluoride 101by plasma are incorporated into the film being deposited on the surfaceof the substrate 5 as shown in FIG. 10A. Further, since the bias voltagehas a duration in which a positive bias is provided periodically,fluorine ions 103 are prevented from being eliminated from magnesiumfluoride 101 deposited on the substrate surface. As a result, theabsorption coefficient of the magnesium fluoride film to be deposited onthe substrate 5 is prevented from increasing in the visible lightregion.

[0109] The effects of the present invention are described below.

[0110]FIG. 11 is a graph showing the dependence of the light absorptanceof a magnesium fluoride thin film upon the pulse frequency of a biasvoltage. In FIG. 11 the horizontal axis and the vertical axis representa wavelength and a light absorptance, respectively. The pulse frequencyof a bias voltage of the waveform shown in FIG. 1B was varied to assumetwo levels of 65 kHz and 100 kHz. As a result, the thin film depositedusing the bias voltage having a pulse frequency of 100 kHz exhibited aconsiderably lowered absorptance as compared with the absorptance of thethin film deposited using the bias voltage having a pulse frequency of65 kHz. Thus, the absorptance obtained with the pulse frequency of 100kHz was satisfactory. Though not specifically described in the subjectexample, the absorptance obtained with the pulse frequency of 65 kHzmade substantially no difference from that obtained without applicationof a positive bias. FIG. 12 to be specifically described later shows thecase where the pulse frequency of a bias voltage was 350 kHz and aquartz (silica) substrate was used. Though the case where the pulsefrequency is more than 350 kHz is not shown, it is theoreticallypreferred that the bias voltage have short positive bias durations witha shortest-possible cycle. Accordingly, a higher pulse frequency is morepreferable. However, it is practically desirable that the pulsefrequency be not higher than 2.45 GHz because too high a pulse frequencymakes plasma discharge in the vacuum chamber unstable. If the pulsefrequency is set to 2.45 GHz, it is desirable to use an ECR (electroncyclotron resonance) apparatus. A film deposition system employing suchan ECR apparatus will be described later.

[0111]FIG. 12 is a graph showing the light absorptance of a magnesiumfluoride thin film deposited on a quartz substrate within the visiblelight region. In this case the magnesium fluoride thin film exhibited alower absorptance than in the case of FIG. 11 where the magnesiumfluoride film was deposited on a glass substrate by virtue of a combinedeffect of the use of the quartz substrate and the setting of the pulsefrequency of a bias voltage to 350 kHz. For reference, the absorptanceof quartz is also shown in FIG. 12.

[0112]FIGS. 13A and 13B are drawings for illustrating a wear-resistancetest; specifically, FIG. 13A is a perspective view schematically showingthe overview of a wear-resistance tester, while FIG. 13B is a tableshowing wear-resistance evaluation criteria.

[0113] As shown in FIG. 13A wear-resistance tester 201 used in this testcomprises a reciprocating movable base 202 for receiving a test sample(a substrate deposited with a thin film) 203 thereon, and a presselement 205 having steel wool 204 on the underside thereof for pressingthe test sample 203 at a predetermined load (700 g in this case).Reference numeral 206 denotes an arm holding the press element 205. Thesteel wool 204 used here was of #0000. The wear-resistance of each testsample was rated on the following four ranks A to D as shown in FIG.13B: rank A indicating a film condition with no flaw, rank B indicatinga film condition with slight flaw, rank C indicating a film conditionwith flaw and peeling, and rank D indicating a film condition almostpeeled.

[0114] According to the results of rating with the wear resistancetester 20 and the wear-resistance evaluation criteria, thewear-resistance of a thin film deposited under application of a positivebias was rated higher by about one rank than that of a thin filmdeposited without application of a positive bias. The wear-resistance ofa thin film deposited with the degree of vacuum optimized in thefilm-depositing process was rated higher by about one to about two ranksthan that of a thin film deposited under usual conditions.

[0115]FIG. 14 is a table showing the crystal grain diameters ofmagnesium fluoride films. As seen from FIG. 14, the crystal graindiameter of a magnesium fluoride thin film deposited by evaporation at300° C. is 12 to 20 nm. A magnesium fluoride thin film deposited byevaporation at a normal temperature (that is, the substrate was notheated) was not crystallized and, hence, determination of the crystalgrain diameter thereof is impossible. In contrast, the crystal graindiameter of a magnesium fluoride thin film deposited using the filmdeposition system according to the subject example is 3 to 10 nm. Fromthis fact it is found that the magnesium fluoride thin film according tothe subject example has a crystal grain diameter that cannot be achievedby the prior art. Conceivably, the main reason therefor is that themagnesium fluoride thin film according to the subject example isdeposited by ion plating at a relatively low temperature (about 100° C.or below).

[0116] Preferable gas species for use in the present invention are asfollows. In the subject example the vacuum chamber 1 is charged withargon gas. Use of a fluorine-containing gas such as CF₄ or SF₆ is moreadvantageous because such a gas can replenish the film to be depositedon the substrate with fluorine.

[0117] The following description is directed to an application of thepresent invention to a multi layered film.

[0118]FIGS. 15A and 15B are drawings for illustrating an application ofthe subject example to a multi layered film; specifically, FIG. 15A is aview showing the structure of a multi layered film, while FIG. 15B is agraph showing the reflectivity of a multi layered film formed on asubstrate within the visible light region. As shown in FIG. 15A, a multilayered film according to the subject example comprises an Al₂O₃ film(refractive index n=1.63), a ZrO₃ film (refractive index n=2.00) and anMgF₂ film (refractive index n=1.38) which are sequentially stacked on asubstrate in that order. This multi layered film had a favorablereflectance as shown in FIG. 15B.

[0119] Next, description will be made of another mode of practicing thepresent invention. FIG. 4 is a schematic overview of the configurationof film deposition system 35 capable of practicing the invention inanother mode. The film deposition system 35 is configured to form filmson the basis of ion plating as a film deposition method.

[0120] The film deposition system 35 includes a vacuum chamber 36, aradio frequency power supply (RF) 37 and a bias power supply unit (DC)38.

[0121] In an upper portion of the inside of the vacuum chamber 36 isdisposed a substrate holder 39 for holding a substrate 5 on its backsurface opposite from its obverse side on which a film is to bedeposited. In the film deposition system 35 shown in FIG. 4 thesubstrate holder 39 is configured so that it is capable of functioningalso as a bias supply electrode for supplying a bias voltage into thevacuum chamber 36. The substrate holder 39 is formed of anelectrically-conductive material so as to be applied with the biasvoltage outputted from the bias power supply unit 38.

[0122] The bias power supply unit 38 is capable of outputting a DCvoltage having a constant positive voltage value. Application of thebias voltage outputted from the bias power supply unit 38 to thesubstrate holder 39 allows negatively-charged ions of a halogen elementto be incorporated into the film being deposited on the substrate 5 aswill be described later.

[0123] In a lower portion of the inside of the chamber 36 is disposed anevaporation source 3 for holding a film material comprising a halogencompound and for evaporating it into the space defined in the chamber36. The evaporation source 3 is of the same construction as that used inthe film deposition system 10 described earlier.

[0124] Between the evaporation source 3 and the substrate 5 in thechamber 36 is disposed an ionization electrode 40 in the form of a coil.The coiled ionization electrode 40 is connected to the radio frequencypower supply 37 and, hence, it is possible to supply a radio frequencypower into the chamber 36 through the ionization electrode 40.

[0125] The radio frequency power supply 37 supplies a radio frequencypower for ionizing the film material evaporated from the evaporationsource 3 and passing through the inside of the ionization electrode 40to generate plasma. The radio frequency power supply 37 has one outputterminal connected to the ionization electrode 40 through a matching box41 and other output terminal grounded.

[0126] The matching box 41 includes a well-known matching circuitcomprising a capacitor and a coil. Adjustment of the matching box allowsmatching to be made between the impedance of the radio frequency powersupply 37 side and that of the vacuum chamber 36 side.

[0127] The vacuum chamber 36 is provided with evacuation means such as avacuum pump and gas supply means, which are not particularly shown, forproviding a desired vacuum atmosphere therein so that the atmospherewithin the vacuum chamber 36 can be adjusted to a desired vacuumatmosphere meeting film-depositing conditions established.

[0128] With use of the film deposition system 35 described above a filmcan be deposited in the following manner. The evaporation source 3 isloaded with a film material comprising a halogen compound, and thesubstrate 5 is set on the substrate holder 39.

[0129] Subsequently, the radio frequency power supply 37 is actuated tosupply a radio frequency power into the chamber 36 through theionization electrode 40, while the halogen compound is evaporated by theevaporation source 3. At the same time, the bias power supply unit 38 isactuated to apply a positive DC voltage to the substrate holder 39.

[0130] In this way, the evaporated halogen compound passing through theionization electrode 40 can be ionized to generate plasma, and resultingions of the halogen compound are allowed to impinge upon and deposit onthe substrate 5 thereby depositing the film.

[0131] Though the halogen element with weak chemical bond in the ionizedhalogen compound, is easily dissociated from the ionized halogencompound in the film depositing process performed by the film depositionsystem 35, it is possible to incorporate the halogen element oncedissociated into the film being deposited on the substrate 5.Specifically, application of the DC voltage outputted from the biaspower supply unit 38 to the substrate holder 39 causesnegatively-charged ions of the halogen element to be incorporated intothe film being deposited on the substrate 5.

[0132] Yet another mode of practicing the present invention is describedbelow. FIG. 5 is a schematic overview of the configuration of filmdeposition system 45 capable of practicing the invention in yet anothermode. The film deposition system 45 is configured to deposit films withuse of a cluster ion beam.

[0133] The film deposition system 45 includes a vacuum chamber 46 and abias power supply unit (DC) 48. In an upper portion of the inside of thevacuum chamber 46 is disposed a substrate holder 49 for holding asubstrate 5 on its back surface opposite from its obverse side on whicha film is to be deposited. In the film deposition system 45 shown inFIG. 5 the substrate holder 49 is configured so that it is capable offunctioning also as a power supply electrode for supplying a biasvoltage into the vacuum chamber 46. The substrate holder 49 is formed ofan electrically-conductive material so as to be applied with the biasvoltage outputted from the bias power supply unit 48.

[0134] The bias power supply unit 48 is capable of outputting a DCvoltage having a constant positive voltage value. Application of thebias voltage outputted from the bias power supply unit 48 to thesubstrate holder 49 allows negatively-charged ions of a halogen elementto be incorporated into the film being deposited on the substrate 5 aswill be described later.

[0135] Cluster ion generating means is disposed in the chamber 46 so asto face opposite to the substrate 5. As will be described below, thecluster ion generating means comprises well-known means for generating acluster of atoms or molecules and well-known means for ionizing thecluster.

[0136] The cluster ion generating means has a cluster evaporation source50 located in a lower portion of the inside of the chamber 46 and acoiled ionization electrode 53. The cluster evaporation source 50 has acrucible for holding therein a film material comprising a halogencompound and a coiled bombard filament surrounding the outer peripheryof the crucible for heating the film material in the crucible.

[0137] As the bombard filament heats the crucible, the film materialupwardly evaporated from a nozzle provided at the upper end of thecrucible becomes clustered and is shot toward the substrate 5. Thiscluster is formed by cooling due to adiabatic expansion in the processof shooting of the film material from the nozzle of the crucible intothe vacuum atmosphere and comprises hundreds or thousands of atoms ormolecules of the film material loosely bound with an intermolecularforce or the like.

[0138] The cluster shot out of the cluster evaporation source 50 isionized by a radio frequency power supplied thereto through theionization electrode 53 during its passage through the inside of theionization electrode 53, thus generating clustered ions.

[0139] The vacuum chamber 46 is provided with evacuation means such as avacuum pump and gas supply means, which are not particularly shown, forproviding a desired vacuum atmosphere therein so that the atmospherewithin the vacuum chamber 46 can be adjusted to a desired vacuumatmosphere meeting film-depositing conditions established.

[0140] With use of the film deposition system 45 described above a filmcan be deposited in the following manner. The cluster evaporation source50 is loaded with a film material comprising a halogen compound, andsubstrate 5 is set on the substrate holder 49.

[0141] Subsequently, a cluster is shot upwardly from the clusterevaporation source 50 and then passed through the inside of theionization electrode 53 to generate clustered ions, which in turn arecaused to impinge upon and deposit on the substrate 5 to deposit thefilm.

[0142] Though the halogen element with weak chemical bond in the clusterof the ionized halogen compound, is easily dissociated from the ionizedhalogen compound in the film depositing process performed by the filmdeposition system 45, it is possible to incorporate the halogen elementonce dissociated into the film being deposited on the substrate 5.Specifically, application of a DC voltage outputted from the bias powersupply unit 48 to the substrate holder 49 causes negatively-charged ionsof the halogen element to be incorporated into the film being depositedon the substrate 5.

[0143] The bias voltage outputted from the bias power supply unit 38 ofthe film deposition system 35 or from the bias power supply unit 48 ofthe film deposition system 45 described above may comprise a positivebias assuming a positive voltage and a negative bias assuming a negativebias.

[0144] That is, the bias voltage used in the film deposition system 35or 45 may be the bias voltage comprising a positive bias assuming apositive voltage value in the form of pulses and a negative biasassuming a constant negative voltage value, which has been described asan example of the bias voltage for use in the film deposition system 10.

[0145] By application of such a bias voltage having a negative voltageportion to the substrate holder 39 or 49 in the film deposition system35 or 45 it is possible to accelerate the movement of positively-chargedhalogen compound ions toward the substrate 5, thereby to deposit a filmof a denser structure on the substrate 5.

[0146] Still another mode of practicing the present invention isdescribed below. FIG. 6 is a schematic overview of the configuration offilm deposition system 60 capable of practicing the invention in stillanother mode. The film deposition system 60 is configured to depositfilms on the basis of ion plating.

[0147] The film deposition system 60 shown in FIG. 6 is of the sameconfiguration as the film deposition system 10 shown in FIG. 1A exceptthe provision of an electron beam evaporation source 55 in the vacuumchamber 1. Members other than the electron beam evaporation source 55are the same as corresponding members used in the film deposition system10. That is, the film deposition system 60 includes vacuum chamber 1,substrate 5, evaporation source 3, and substrate holder 2 serving alsoas a bias voltage supply electrode.

[0148] Reference numeral 8 denotes an electric power supply unitcomprising radio frequency power supply unit 11 and bias power supplyunit 12. The bias power supply unit 12 comprises waveform generator 13and bias power supply 14. Reference numeral 15 denotes a high passfilter, while reference numeral 16 denotes a first low pass filter.

[0149] These members provided in the film deposition system 60 are eachconstructed similarly to the corresponding member of the film depositionsystem 10 to operate in the same manner as does the correspondingmember.

[0150] The electron beam evaporation source 55 is a well-known electronbeam evaporation source capable of evaporating a film material byheating with an electron beam. The electron beam evaporation source 55includes a crucible 56 for holding a halogen compound as the filmmaterial. The crucible 56 is loaded with the same material as thehalogen compound stored in the evaporation source 3.

[0151] The halogen compound in the crucible 56 is heated with electronbeam 59 emitted from an electron gun not shown and is evaporated from anopening defined at an upper end of the crucible 56 into a spaceextending above the crucible 56.

[0152] The electron beam evaporation source 55 is provided with ashutter 57 spaced a predetermined distance above the upper end of thecrucible 56 and disposed to cover the crucible 56. The shutter 57 canrevolve about a support shaft 55 a relative to the crucible 56 to switchits position between a closing position covering the crucible 56 fromabove and an open position retreated away from the position above thecrucible 56.

[0153] When the shutter 57 is in the closing position, the shutter 57obstructs the flow of the halogen compound evaporated from the crucible56 and, hence, the evaporated halogen compound cannot directly impingeupon the substrate 5.

[0154] In the deposition of a film on the substrate 5 with use of thefilm deposition system 60 the halogen compound in the crucible 56 isheated, with the shutter 57 of the electron beam evaporation source 55being in the closing position.

[0155] By so doing, it becomes possible to evaporate the halogencompound stored in the crucible 56 from the crucible 56. Since theshutter 57 obstructs the flow of the halogen compound thus evaporated,the evaporated halogen compound cannot directly advance toward thesubstrate 5 and hence is guided around sidewardly of space 58 definedbetween the crucible 56 and the shutter 57 into an open space 1 aextending to the substrate 5 within the chamber 1.

[0156] Since ions of the halogen element are easily dissociated from theevaporated halogen compound, it is possible to guide such dissociatedhalogen ions to the substrate 5 by the positive voltage portion of thebias voltage outputted from the bias power supply unit 12.

[0157] With the film deposition system 60 thus configured to evaporatethe halogen compound by means of the electron beam evaporation source 55and obstruct the direct impingement of the halogen compound evaporatedby the evaporation source 55 upon the substrate 5 by means of theshutter 57, ions of the halogen element dissociated from the evaporatedhalogen compound can be supplied to the substrate 5 preferentially.

[0158] Thus, it becomes possible to increase the amount of halogen ionsto be supplied to the substrate 5 relative to the amount of the halogencompound evaporated in the chamber 1, thereby to avoid a relativedecrease in the amount of the halogen contained in the film to bedeposited on the substrate 5.

[0159] Since the evaporation source 55 evaporates the halogen compoundusing an electron beam, the evaporation source 55 is capable ofevaporating the halogen compound in a more finely decomposed state, thusallowing ions of the halogen element to be dissociated more easily.Therefore, it becomes possible to supply such halogen ions to thesubstrate 5 more easily.

[0160] While the foregoing description has been directed to an exampleof the method of supplying halogen element ions to the substrate 5 bymeans of the electron beam evaporation source 55 with use of the filmdeposition system 60 comprising the electron beam evaporation source 55in addition to the film deposition system 10 described with reference toFIG. 1A, it is possible to employ any other vacuum film depositionmethod which is capable of depositing films by ionizing a film material.

[0161] For example, it is possible to add the aforementioned electronbeam evaporation source 55 to the film deposition system 35 describedwith reference to FIG. 4 or the film deposition system 45 described withreference to FIG. 5. In such a case also, ions of the halogen elementcan be supplied to the substrate 5 preferentially rather than thehalogen compound evaporated by the evaporation source 55.

[0162] To be described below is still yet another mode of practicing thepresent invention.

[0163]FIG. 16 is a schematic view showing the configuration of a vacuumfilm deposition system that is suitable for a bias voltage having ahigher pulse frequency.

[0164] As shown in FIG. 16, the vacuum film deposition system of thisconfiguration includes an ECR apparatus 613 instead of the combinationof the radio frequency power supply unit and the high pass filter shownin FIG. 1A. The ECR apparatus 613 has an ECR cavity 607 open at a wallportion of the vacuum chamber 1 and an ECR power supply 608 and isconfigured to generate high-density plasma by directing a microwave of2.45 GHz generated by the ECR power supply 608 into the ECR cavity 607to cause electron cyclotron resonance under application of a magneticfield by a magnet not shown. Such plasma 611 is supplied into the vacuumchamber 1. Other features are the same as the corresponding ones of thefilm deposition system shown in FIG. 1A. This vacuum film depositionsystem employs evaporation source 603 of the resistive heating typeadapted to evaporate a thin film material (magnesium fluoride) 606placed on a boat 603 a by resistive heating. Behind the substrate holder2 is disposed a heater 605 for heating the substrate 5 from behind.Reference numeral 604 denotes a film thickness sensor, while referencenumeral 609 denotes a gas outlet. Further, reference numeral 610 denotesthe film material in an evaporated state.

[0165] The vacuum film deposition system thus configured supplies plasma611 into the vacuum chamber 1 by means of the ECR apparatus 613 andhence prevents electric discharge from becoming unstable even if thepulse frequency of the bias voltage is made relatively high. It ispossible to raise the pulse frequency up to a maximum of 2.45 GHz, whichis equal to the frequency of the microwave supplied from the ECR powersupply 608.

[0166] To be described below is yet still another mode of practicing thepresent invention. A vacuum film deposition system configured for use inthis mode is similar to the vacuum film deposition system shown in FIG.1A except that the bias power supply unit 12 and the low pass filter 16are not provided and that the resistor and the capacitor used in amatching box not shown have respective predetermined values. Thismatching box comprises a fixed capacitor, a fixed resistor, a variablecapacitor and the like. The inventor of the present invention has foundthat the following phenomenon occurs when the fixed capacitor and thefixed resistor have respective predetermined values.

[0167]FIG. 18 is a graph showing variations in the electric potential ofthe substrate holder 2 shown in FIG. 1A. When the radio frequency powersupply unit of the subject vacuum film deposition system is actuated tosupply a radio frequency power into the vacuum chamber 1, the electricpotential of the substrate 2 varies like an oscillating wave thatoscillates with an amplitude V_(a) from a substantially constantnegative voltage V_(dc) serving as a center. The frequency of thisoscillating wave is equal to or an integer multiple of the frequency(usually 13.56 MHz) of a radio frequency power outputted from the radiofrequency power supply unit. The negative voltage V_(dc) is consideredto be a voltage corresponding to a self-bias that is generated in ausual case. At present, the mechanism based on which such a phenomenonoccurs has not been elucidated yet. Since the amplitude V_(a) of theoscillating wave is slightly larger than the negative voltage V_(dc) inthis vacuum film deposition system, the electric potential of thesubstrate holder 2 varies with a frequency equal to the frequency of aradio frequency power and periodically assumes a positive potential fora duration Δt. Thus, the subject vacuum film deposition system iscapable of depositing a halogen compound thin film with lesseneddeficiency of the halogen element like the vacuum film deposition systemshown in FIG. 1A. It should be noted that though the waveform of theforegoing oscillating wave in FIG. 18 is shown as a train of positiveand negative alternate pulses, it is actually a sinusoidal wave. Thepositive potential duration Δt is preferably 10 μs or less. On the otherhand, the frequency of the oscillating wave is preferably within therange of from 13.56 MHz to 4×13.56 MHz.

[0168] As described above, the present invention is capable ofdepositing a film through ionization of a halogen compound as a filmdeposition system while allowing ions of the halogen element dissociatedfrom ions of the halogen compound to be incorporated into the film beingdeposited, thereby making it possible to prevent the film from becomingdeficient in the halogen element.

[0169] Thus, the present invention is capable of depositing a filmthrough ionization of a halogen compound while preventing the film frombecoming deficient in the halogen element, whereby the film thusdeposited can be rendered dense and firm without impairment of thedesired function.

[0170] Further, the present invention is capable of preventing anintended film from becoming deficient in a halogen element, which is noteasy to supplement in the form of a reactive gas to be supplied into thevacuum chamber. In depositing a film of an oxide for example, it ispossible to prevent the film from becoming deficient in oxygen bysupplying oxygen gas to the film being deposited in the vacuum chamber.With the present invention, however, there is no need to supplement ahalogen in the form of a reactive gas.

[0171] As described above, the present invention provides the effect ofdepositing a halogen film while preventing the film from becomingdeficient in the halogen element thereby ensuring the halogen compoundfilm without impairment of its desired function.

[0172] As this invention may be embodied in several forms withoutdeparting from the spirit of essential characteristics thereof, thepresent embodiments are therefore illustrative and not restrictive,since the scope of the invention is defined by the appended claimsrather than by the description preceding them, and all changes that fallwithin metes and bounds of the claims, or equivalence of such metes andbounds thereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A film deposition method for depositing a halogencompound film, comprising the steps of: placing a substrate on anobverse side of a bias supply electrode disposed in a vacuum chamber;evaporating a film material comprising a halogen compound; supplying aradio frequency voltage to the bias supply electrode used as one ofelectrodes to generate plasma in the vacuum chamber; and applying to thebias supply electrode a bias voltage varying in the form of a wavehaving a negative mean value and a maximum value exceeding a self-biascaused by the radio frequency voltage, whereby the evaporated filmmaterial is ionized and deposited on the substrate to deposit thehalogen compound film on the substrate.
 2. The method according to claim1, wherein the maximum value of the bias voltage is a positive voltage.3. The method according to claim 1, wherein the bias voltage is appliedby a power supply.
 4. The method according to claim 1, wherein the biasvoltage applied is based on a voltage generated in a matching circuitprovided for matching an impedance on a power supply side associatedwith the radio frequency voltage to that on a vacuum chamber side. 5.The method according to claim 1, wherein:
 6. The method according toclaim 1, wherein the film material comprising the halogen compound isevaporated and ionized in a clustered fashion by cluster ion generatingmeans.
 7. The method according to claim 1, wherein a substrate holderholding the substrate on its back surface in the vacuum chamber isformed of an electrically-conductive material and is used also as thebias supply electrode.
 8. The method according to claim 1, wherein: anelectron beam evaporation source is further provided comprising acrucible holding a material same as the halogen compound to beevaporated, an electron gun for evaporating the halogen compound held inthe crucible by heating with an electron beam, and a shutter spaced apredetermined distance from the crucible in a direction toward thesubstrate; and the halogen compound in the crucible is evaporated byheating with the electron beam, while direct impingement of the halogencompound thus evaporated upon the substrate is obstructed with theshutter, whereby the evaporated halogen compound is guided sidewardly ofthe shutter and the crucible.
 9. The method according to claim 1,wherein the halogen compound is magnesium fluoride (MgF₂).
 10. Themethod according to claim 1, wherein the bias voltage has a frequencyranging between 100 kHz and 2.45 GHz.
 11. An apparatus for forming ahalogen compound film, comprising: a vacuum chamber; a bias supplyelectrode disposed in the vacuum chamber and having an obverse side forreceiving a substrate thereon; an evaporation source for evaporating afilm material of the halogen compound film to be formed on thesubstrate; a radio frequency power supply for supplying a radiofrequency voltage to the bias supply electrode used as one of electrodesto generate plasma in the vacuum chamber; and a bias power supply forapplying to the bias supply electrode a bias voltage having a frequencyranging between 100 kHz and 2.45 GHz and varying in the form of a wavehaving a negative mean value and a positive maximum value.
 12. Amagnesium fluoride film obtainable by a method comprising the steps of:placing a substrate on an obverse side of a bias supply electrodedisposed in a vacuum chamber; evaporating magnesium fluoride; supplyinga radio frequency voltage to the bias supply electrode used as one ofelectrodes to generate plasma in the vacuum chamber; and applying to thebias supply electrode a bias voltage having a frequency ranging between100 kHz and 2.45 GHz and varying in the form of a wave having a negativemean value and a positive maximum value, thereby forming the magnesiumfluoride film on the substrate.
 13. A magnesium fluoride film having acrystal grain diameter not less than 3 nm and not more than 10 nm.